Additive manufacturing—so called “3D printing”—is the term given to processes that manufacture objects via sequential-layer material addition/joining throughout a 3D work envelope under automated control. ISO/ASTM52900-15 defines seven categories of additive manufacturing processes: binder jetting, directed energy deposition, material extrusion, material jetting, powder bed fusion, sheet lamination, and vat polymerization.
Composite 3D printing processes produce an object or part (hereinafter collectively “part”) by depositing composite filaments made of continuous fiber-reinforced thermoplastics, which quickly harden to form a layer. Successive layers of material are deposited to build the object.
A fiber composite consists of fibers as the discontinuous phase, a matrix (typically a resin) as the continuous phase, and an interface region (the interface). The strength of a fiber composite is determined by the wettability of the fiber within the resin matrix. Wettability refers to the degree of adhesion and bonding between the resin and the fibers. A high degree of wetting means the resin has good surface adhesion to the fibers, coating the fibers uniformly. A low degree of wetting means the fibers are not completely coated by resin, resulting in void space or bubbles between the fiber and resin.
When subjected to stress, a composite part with a high degree of wetting will effectively transfer the stress to the fibers through the resin matrix. This composite part then has a much higher tensile strength as compared to a part made only of resin material. In comparison, a composite part with a low degree of wetting will experience fiber pullout when subjected to stress. Fiber pullout results in ineffective stress transfer from the resin matrix to the fibers due to the void space at the fiber/resin interfaces. The void space acts as nucleating sites for cracks, resulting in premature failure of the part via crack propagation. The composite part with a low degree of wetting has a lower tensile strength as compared to a part made only of the resin material due to the higher void space with this composite part.
To ensure a high degree of wetting, the composites industry mostly uses thermoset (e.g., epoxy, etc.) resin systems. Thermosets typically have a low viscosity compared to other polymers, resulting in high melt flow and a high degree of fiber wetting. While thermoset resins provide sufficient wetting, they cannot tolerate higher operating temperatures, often limited to 100° C. or less. As compared to thermosets, thermoplastics generally have a higher viscosity, making it more difficult to achieve sufficient fiber wetting; however, thermoplastics offer higher tensile strength and stiffness, higher toughness, and a higher operating temperature as compared to thermoset resins.
U.S. patent application Ser. No. 14/184,010 discloses an apparatus for manufacturing an object having a deposition head including a nozzle for heating thermoplastic composite material having one or more fiber strands. In some embodiments, the apparatus also includes a turntable, a robotic arm for moving the deposition head, and spools of composite filament.
That invention involved the use of a heated extrusion nozzle to melt and deposit thermoplastic fiber-composite filament. The composite feed filament comprises thousands (e.g., 1K, 3K, 6K, 12K, 24K, etc.) of continuous fibers impregnated with thermoplastic resin. The continuous fiber includes, without limitation, carbon, fiberglass, aramid (AKA Kevlar), and carbon nanotubes (CNT). The composite filament has a cylindrical, elliptical, or rectangular cross section. In the case of a rectangular cross section, the aspect ratio (width-to-thickness) is about 3:2; that is, it is distinguishable from a tape, which is significantly wider than it is thick.
That deposition process completely melts the thermoplastic filament as it contacts the nozzle. As the continuous filament is pushed through the nozzle, the pressure of the nozzle forces the polymer underneath the continuous carbon fiber tow, which can result in exposed fiber on the top surface. As a result, there is a loss of fiber adhesion within the polymer matrix and the wetting of the fibers by the polymer is reduced.
In addition to a loss of wetting, this type of deposition involves a 90-degree bend between the nozzle and part being manufactured, which can cause fiber breakage and damage. More specifically, as the fibers bend around a 90-degree angle in the melt phase, the fibers become twisted and tangled together. The twisting and tangling of the fibers can cause them to break, reducing the overall strength and stiffness of the deposited material. The twisting and tangling further reduces the wetting of the fibers and also produces an uneven, rough surface finish.
Therefore, a need exists for way to improve the mechanical properties, surface fidelity, and finish of additively manufactured thermoplastic fiber composite parts.